In a next generation wireless communication system, it is expected that a relay station (RS) will be widely used. The concept of the RS will be described in brief as follows.
A standardization project of a new subject entitled “a multi-hop relay” is in progress now after publication of a standard for fixed subscriber terminals, i.e., IEEE 802.16-2004, and a standard to provide mobility of subscriber terminals, i.e., IEEE 802.16e-2005, in Institute of Electrical and Electronics Engineers (IEEE) 802.16 in 2006.
In such a project which Task Group J (IEEE 802.16j) in IEEE 802.16 takes part in, from the first official conference was carried out in May, 2006, at the second conference in July, 2006, a usage model, terminology and technical requirements started to be discussed in earnest. Hereinafter, Task Group J in IEEE 802.16 will be abbreviated to “802.16j”.
The concept of the relay station which will be described hereinafter may be substantially identically used in the case of a relay station considered in a 3GPP LTE-A system. Further, relay stations having the same or similar functions in various other wireless access systems may be used to have the similar concept as the relay station described in the present invention.
A Project Authorization Request (PAR) of 802.16j which is a standardization operation which will progress in the future has two objects, i.e., coverage extension and throughput enhancement.
Relay stations may be generally divided into two kinds, a transparent kind and a non-transparent kind. A transparent relay station has all operations and functions present therein, and manages terminals. On the other hand, a non-transparent relay station serves to relay all operations and functions between a macro base station and a terminal.
From the viewpoint of a terminal, the terminal treats the transparent relay station and the non-transparent relay station as one macro base station without distinction and does not cause any change of an operation, but may have a function of discriminating between a relay station and a macro base station.
A network including a relay station may include a base station (BS), the relay station (RS) and a terminal (mobile station: MS). The MS may receive a wireless signal through the RS even at the outside of a cell area of the base station. Further, a path of a high quality having a high-level adaptive modulation and coding (AMC) method of the MS at the inside of the cell area of the base station may be set through the RS. Therefore, a user may obtain an effect of increasing capacity of the overall system using the same wireless resources.
A standard which will be made by the 802.16j project has designated requirements. For example, mobile stations implemented based on the IEEE 802.16-2004 standard and the IEEE 802.16e-2005 standard should communicate with a relay station without addition of any function. Therefore, in the conventional system, the scope of application of the relay station may be restricted in the form of addition of some functions to control the relay station to the relay station and the existing base station. It is expected that the standard of the relay station will be a key issue of standardization from now on.
The relay station may be considered as a kind of subscriber terminal which performs operations of a physical layer and a medium access control layer, and may be mainly controlled by a base station but may have a designated control function as needed. In a usage model under discussion, various types of relay stations, such as a mobile relay station to provide a temporary service to a specific region and a relay station installable in a vehicle, a subway, etc. as well as a fixed relay station are considered.
Representative technical issues which will be discussed later are as follows.
1. A procedure through which a base station identifies relay stations present in the area thereof and obtains and maintains information regarding topology with the relay stations.
2. A definition of a physical transmission frame between a relay station and a mobile station having backward compatibility with the conventional IEEE 802.16 system
3. A signal procedure to provide mobility between relay stations or between a relay station and a base station.
4. A network entry procedure of a relay station into a base station and an entry procedure of a mobile station through a relay station.
Frame structures used in relay stations may include a downlink frame structure and an uplink frame structure. Here, the downlink frame structure may include a downlink (DL) access zone and a DL relay zone, and the uplink frame structure may include an uplink (UL) access zone and a UL relay zone.
If only one relay station (one hop structure) is present between a base station (ABS) and a mobile station (AMS), the downlink access zone represents a section in which the ARS transmits a data packet to the AMS or other lower relay stations, and the uplink access zone represents a section in which the AMS or other lower relay stations transmit a data packet to the corresponding ARS. Further, the ARS in the downlink relay zone may receive a data packet from the ABS, and the ARS in the uplink relay zone may transmit a data packet to the ABS.
The data packet may have the form of a medium access control protocol data unit (MAC PDU). In order to assist understanding of the MAC PDU, a protocol layer model defined in a general broadband wireless access system will be described first.
FIG. 1 is a diagram illustrating a protocol layer model defined in a generally used wireless mobile communication system based on an IEEE 802.16 system.
With reference to FIG. 1, a MAC layer belonging to a link layer may include three sublayers. First, a service-specific convergence sublayer (CS) may deform or map data of an external network received through a CS service access point (SAP) into MAC service data units (SDUs) of a MAC common part sublayer (CPS). Such a sublayer may include a function of discriminating SDUs of the external network and then assigning corresponding MAC service flow identifiers (SFIDs) and connection identifiers (CIDs) to the SDUs.
Next, the MAC CPS is a sublayer providing essential functions of the MAC, such as system access, bandwidth allocation, connection setting and management, and receives data classified by connection of a specific MAC from various CSs through the MAC SAP. Here, quality of service (QoS) may be applied to data transmission and scheduling through the physical layer.
Further, a security sublayer may provide authentication, security key exchange and encryption functions.
The MAC layer may be implemented as the concept of transport connection through a connection-oriented service. When a mobile station is registered in the system, a service flow may be prescribed by negotiation between the mobile station and the system. If service requirements are changed, a new connection may be set. Here, transport connection defines mapping between peer convergence processes using the MAC and the service flow, and the service flow defines QoS parameters of the MAC PDU exchanged in the corresponding connection.
The service flow on the transport connection performs an essential role in operation of the MAC protocol, and provides a mechanism for QoS management of the uplink and the downlink. Particularly, the service flows may be connected to a bandwidth allocation process.
In the general IEEE 802.16 system, a mobile station may have a universal MAC address having a length of 48 bits per wireless interface. Such an address defines the wireless interface of the mobile station, and may be used to set connection of the mobile station during an initial ranging process. Further, since the base station verifies mobile stations through different IDs of the respective mobile stations, the universal MAC address may be used as a part of the authentication process.
The respective connections may be identified by connection identifiers (CIDs) having a length of 16 bits. While initialization of a mobile station is performed, two pairs of the management connection are set between the mobile station and the base station, and three pairs including the two pairs of the management connection may be selectively used. Recently, in the IEEE 802.16m system, the CIDs may be replaced with station identifiers (STIDs) and flow identifiers (FIDs) to identify the flow. Here, the STID means an identifier of 12 bits which is allotted to a mobile station performing network (re)entry by the base station, and the FID means an identifier of 4 bits to identify the connection (the management connection or the transport connection) to a specific mobile station. An ARS STID may be applied to an advanced relay station (ARS) of an IEEE 802.16m system.
Under the above-described layer structure, a transmitting end and a receiving end may exchange data or a control message through the MAC PDUs. In order to generate such MAC PDUs, the base station or the mobile station may allow the MAC PDU to include a MAC header.
That is, the MAC PDU may include the MAC header, an extended header and a payload. The MAC PDU may include the MAC header at any time, and may selectively include the payload as needed. However, the MAC PDU does not include the extended header without the payload.
As one method through which the ARS can efficiently relay data from the ABS to a plurality of different terminals (AMSs) or data from the plurality of different AMSs to the ABS, a tunnel mode or a relay mode will be described with reference to FIG. 2.
FIG. 2 illustrates one example of a connection state between mobile stations (AMSs), a base station (ABS) and a relay station (ARS) to which the tunnel mode is applicable.
The tunnel mode refers to a mode in which the ARS forms tunnels and transmits MPDUs through the tunnels if the ARS relays data between the ABS and the plural AMSs, as shown in FIG. 2.
The ARS connected to the ABS may be uniquely discriminated in the area of the ABS by an ARS STID. If the tunnel mode is applied, the respective tunnels formed between the ABS and ARS may be discriminated from each other through different FIDs. That is, respective tunnel connections may be uniquely discriminated through combinations of the ARS STID and FIDs.
Two or more MPDUs transmitted to the plural AMSs or from the plural AMSs may be packed together with one relay MAC PDU, or may be connected to the relay MAC PDU and transmitted to a relay link. Here, which terminals the respective MPDUs are transmitted to may be determined by STIDs. For this purpose, STID information of the MPDUs may be included in the relay MAC PDU. The ARS uses the STID information included in a downlink relay MAC PDU to generate an A-MAP in an access link, and the ABS indicates which terminals the respective MAC PDUs belong to using the STID information included an uplink relay MAC PDU.
When the ARS has finished network entry into the ABS, one or more tunnels may be generated. Connection to one terminal may be mapped into one or more tunnels. In the tunnel mode, the MAC PDUs transmitted through the tunnel together with relay MAC headers including tunnel identifiers (i.e., the FIDS) may be encapsulated into the relay MAC PDU. That is, the plural MAC PDUs transmitted through one tunnel may be connected into one relay MAC PDU, and be transmitted.
Next, an ARQ technique in the relay environment will be described.
The ARQ technique refers to a technique through which a transmitting side or a receiving side detects a data error generated in a transmission line, and requests retransmission of data if the data error is detected.
Hereinafter, a general ARQ procedure will be described with reference to FIG. 3.
FIG. 3 is a block diagram illustrating the general ARQ procedure of a transmitting end.
With reference to FIG. 3, a not-sent state is switched to an outstanding state according to beginning of transmission, and when an ACK is received from a receiving end in the outstanding state, the ARQ procedure is completed. If the transmitting end receives ARQ_RETRY_TIMEOUT or NACK from the receiving end in the outstanding state, retransmission is performed. Here, if retransmission succeeds (ACK is received), the ARQ procedure is completed. If a lifetime of an ARQ block (ARQ_BLOCK_LIFETIME) has expired in the outstanding state or the retransmission state, corresponding data is discarded.
In the above-described general ARQ technique, it is assumed that data transmission is carried out between one transmitting end and one receiving end. However, if an RS relays data transmission between the transmitting end and the receiving end, the ARQ procedure is modified. This will be described with reference to FIG. 4.
FIG. 4 is a block diagram illustrating an ARQ procedure if a transmitting end is a base station in a system including a relay station.
The ARQ procedure of FIG. 4 is similar to the general ARQ procedure of FIG. 3, but differs from the general ARQ procedure of FIG. 3 in that the ARQ procedure of a corresponding ARQ block is completed only if ACK (MS-ACK) from a mobile station is received regardless of whether or not ACK (RS-ASK) from a relay station is received.
There are three ARQ modes which are applicable in a multi-hop relay system.
The first ARQ mode is referred to as an end-to-end ARQ mode which is performed between a BS and MS. In such a mode, the additional ARQ capability of an RS is not requested.
The second ARQ mode is referred to as a two-link ARQ mode which is performed between a MR-BS and an access RS and between an access RD and a MS. In such a mode, an ARQ operation is divided according the above-described two links.
The final ARQ mode is referred to as a hop-by-hop ARQ mode which is performed between two adjacent objects (a MS, an RS or a BS). In such a mode, an ARQ operation may be divided according to respective links.
If the hop-by-hop ARQ mode or the two-link ARQ mode is applied to the system including the RS, the RS does not relay MS-NACK but relays only MS-ACK to the BS. Thus, by intervening the RS in the ARQ procedure between the MS and the BS, the RS should delete or amend feedback information (for example, an extended header including ARQ feedback IE) generated from the MS. This increases complexity of the RS, thus lowering efficiency.